A film formed with a metallic paint reflects external incident light with a flaky metallic pigment contained in the film, for exhibiting a glittery design. The reflection of light presents a unique appearance excellent in design along with each color tone of the film.
With such characteristics, a metallic pigment mainly employing aluminum flakes as basic particles is applied to a metallic design for automobile paint finish, plastic paint finish, printing ink or a resin composition molding.
In recent years, a colorful and highly bright metallic pigment colored red, green, purple or the like has been desired following diversification of consumers' preference. However, the originally achromatic aluminum flakes forming a pigment presenting silver-gray luster cannot satisfy the aforementioned consumers' demand as such.
Therefore, many attempts have been made in order to solve these problems and satisfy the consumers' requirement for diverse colors. For example, Japanese Patent Laying-Open No. 58-141248, National Patent Publication No. 5-508424, Japanese Patent Laying-Open No. 1-315470, Japanese Patent Laying-Open No. 9-40885, Japanese Patent Laying-Open No. 9-59532 or Japanese Patent Laying-Open No. 9-124973 discloses a technique related to a color metallic pigment prepared by bonding a color pigment to metal particles.
A method of bonding a color pigment to the surfaces of metal particles and covering the same with coats consisting of a polymer composition is generally employed as a method of preparing such a color metallic pigment. Further, an organic pigment such as a diketopyrrolopyrrole-based, quinacridone-based, dioxazine-based, isoindolinone-based, condensed azo-based, threne-based, perinone-based, perylene-based, phthalone-based or phthalocyanine-based pigment or an inorganic pigment such as iron oxide or carbon black is generally used as the color pigment employed for such a color metallic pigment.
In this color metallic pigment, however, the color pigment bonded to the surfaces of the metal particles is disadvantageously easy to photo-deteriorate due to reflection of light on these surfaces. A pigment such as phthalocyanine green, phthalocyanine blue or iron oxide relatively excellent in light fastness must be selected in order to solve this problem, and hence the design of the obtained color metallic pigment is limited under the present circumstances.
On the other hand, Japanese Patent Laying-Open No. 63-161063, Japanese Patent Laying-Open No. 4-28771 or Japanese Patent Laying-Open No. 2001-316609 discloses a technique of forming a film of silicon oxide, titanium oxide or metal on the surface of a pearlescent pigment such as mica thereby providing a colored nacreous pigment provided with an interference color. However, the hiding power of this colored nacreous pigment is so small that the pigment cannot sufficiently conceal the ground when compounded with a paint or ink.
With respect to these problems, some techniques have been disclosed in relation to a color metallic pigment obtained by coating the surfaces of metal particles having high hiding power with interference films of silicon oxide, aluminum oxide or titanium oxide.
For example, Japanese Patent Laying-Open No. 1-110568 or Japanese Patent Laying-Open No. 2-669 discloses a method of depositing titanium oxide on the surfaces of metal particles by a sol-gel method. According to this method, however, no color metallic pigment having high chroma can be obtained but the titanium oxide layers must be in an active anatase phase, to disadvantageously prompt deterioration of resin and reduce weather resistance when compounded with a paint or the like.
Japanese Patent Laying-Open No. 56-120771, Japanese Patent Laying-Open No. 1-311176 or Japanese Patent Laying-Open No. 6-32994 discloses a method of obtaining a composite phase of iron oxide, titanium oxide or metallic oxide and carbon, metal or metallic oxide on the surfaces of metal particles by a gas phase method. When employing the gas phase method, however, it is necessary to fluidize the metal particles and supply a precursor of metallic oxide for heating/depositing the same, while a specific apparatus is required, the metal particles are remarkably in danger of dust explosion, and the precursor of the metallic oxide is generally hard to handle due to strong toxicity.
Japanese Patent Laying-Open No. 8-209024 discloses a technique related to a multilayer coat metallic pigment based on a two-layer structure of a colorless cover layer having a refractive index of not more than 1.8 and a selective absorption layer having a refractive index of at least 2.0. However, while this technique discloses a method of forming metallic oxide layers on the surfaces of metal particles by the gas phase method or a method of hydrolyzing a metallic compound in a solution, the gas phase method has the aforementioned disadvantage, and the hydrolytic reaction is caused in a basic or acidic atmosphere containing a large quantity of water in the method of hydrolyzing the metallic compound in the solution for forming the metal oxide layers and hence the metal particles may react with the water during a treatment step to disadvantageously result in agglomeration of the metal particles or acceleration of the reaction.
Japanese Patent laying-Open No. 51-150532 or Japanese Patent Laying-Open No. 63-15861 discloses a method of mixing aluminum powder, metallic salt and a chelate compound with each other for depositing a metallic oxide layer. However, the method described in Japanese Patent Laying-Open No. 51-150532, carried out in an aqueous solution with the aluminum powder remarkably in danger of strongly reacting with the solution for the treatment, may unpractically cause a problem such as generation of a large quantity of hydrogen gas, remarkable heat generation resulting from acceleration of reaction or agglomeration of the aluminum powder.
Also in the method described in Japanese Patent Laying-Open No. 63-15861, it is difficult to avoid the aforementioned problem resulting from strong reaction between the aluminum powder and the solution for the treatment, while the reaction is caused in an acidic to neutral region and hence deposition of the metallic compound is not efficiently performed but unreacted metallic salt disadvantageously remains in a large quantity.
The metallic oxide layer obtained by this method, formed by a hydrated film containing water, is inferior in adhesiveness to a binder such as resin contained in the paint and tends to cause a problem such as peeling or reduction of moisture resistance with time. Further, the film of the aforementioned metallic oxide layer formed by the hydrated film tends to be incomplete, and disadvantageously exhibits rather inferior color development in general.